(1) Field of the Invention
The present invention relates to a laser drawing apparatus and laser drawing method for scanning a laser beam over an image-receiving object such as a printed circuit board to draw a desired pattern on the image-receiving object. More particularly, the invention relates to a technique for calibrating a laser beam quantity detecting device in a laser beam quantity control system.
(2) Description of the Related Art
In a conventional laser drawing apparatus, an exposure head is driven to emit a laser beam modulated and deflected based on raster data toward an image-receiving object mounted on a drawing table. The laser beam scans the image-receiving object in a primary scanning direction, while the drawing table is moved in a secondary scanning direction perpendicular to the primary scanning direction. As a result, a desired pattern such as a circuit pattern is formed over an entire imaging area of the image-receiving object.
Generally, the quantity of the laser beam emitted from the exposure head is adjusted by the following two known methods.
In the first method, a beam sampler extracts part of the laser beam having undergone a quantity adjustment by a light quantity adjusting device in the exposure head, and a photo sensor detects the quantity of the extracted part of the laser beam. The light quantity adjusting device is controlled based on the result of detection provided by the photo sensor.
The second method is described in Japanese Patent Publication (Unexamined) 1999-109273. In this method, part of the laser beam emitted from the exposure head outside the imaging area of the image-receiving object is reflected by a mirror, and a photo sensor detects the reflected part of the laser beam. The light quantity adjusting device is controlled based on the result of detection provided by the photo sensor.
The conventional apparatus and methods with the above features have the following drawback.
As noted above, the beam sampler or mirror is used to separate out part of the laser beam, and the quantity of the separated part of the laser beam is detected to adjust the quantity of the laser beam emitted to the image-receiving object. However, a long period of use inevitably lowers the precision in laser drawing on image-receiving objects.
The present invention has been made having regard to the state of the art noted above, and its object is to provide a laser drawing apparatus and laser drawing method which are capable of maintaining a high degree of precision in laser drawing on image-receiving objects.
To fulfill the above object, Inventors have made intensive research and attained the following findings. Part of a laser beam is separated out by an optical system to detect the quantity of the laser beam with a light quantity detecting device. For this purpose, the light quantity detecting device includes an optical component such as a beam sampler or mirror as noted above. A light-receiving portion of such optical component undergoes a gradual deterioration in its coating due to the power of the incident laser beam. Further, fine particles of foreign matter may adhere to the light-receiving portion. The deterioration of the light-receiving portion or the adhesion thereto of foreign matter results in variations in the separation ratio and optical efficiency of the beam sampler, or variations in the reflectivity of the mirror. Such variations are found to have a causal relationship with the inability to detect the quantity of the laser beam accurately, and therefore the laser beam is not emitted in a proper quantity to image-receiving objects, resulting in a low precision of laser drawing on the image-receiving objects.
Based on the above findings, the present invention provides a laser drawing apparatus for drawing a desired pattern on an image-receiving object by scanning the object with a laser beam, the apparatus comprising:
a holding device for holding the image-receiving object;
an exposing device having a light quantity adjusting device for adjusting quantity of the laser beam, an extracting device for extracting at least part of the laser beam in the quantity adjusted by the light quantity adjusting device, and a first light quantity detecting device for detecting quantity of the laser beam extracted by the extracting device, the laser beam in the quantity adjusted by the light quantity adjusting device being scanned in a primary scanning direction over the image-receiving object held by the holding device;
a moving device for moving the holding device relative to the exposing device in a secondary scanning direction perpendicular to the primary scanning direction;
a drawing control device for controlling the exposing device and the moving device;
a second light quantity detecting device disposed on the holding device for directly detecting the quantity of the laser beam emitted from the exposing device toward the holding device; and
a calibrating device for calibrating the first light quantity detecting device based on results of detection by the first light quantity detecting device and results of detection by the second light quantity detecting device.
According to the apparatus of the invention, the exposing device includes a light quantity adjusting device for adjusting quantity of the laser beam, an extracting device for extracting at least part of the laser beam in the quantity adjusted by the light quantity adjusting device, and a first light quantity detecting device for detecting quantity of the laser beam extracted by the extracting device. The laser beam in the quantity adjusted by the light quantity adjusting device is scanned in the primary scanning direction over the image-receiving object held by the holding device. The moving device moves the holding device relative to the exposing device in the secondary scanning direction perpendicular to the primary scanning direction. The drawing control device controls the exposing device and the moving device. The second light quantity detecting device is disposed on the holding device, and directly detects quantities of the laser beam emitted from the exposing device toward the holding device. The calibrating device calibrates the first light quantity detecting device based on results of detection by the first light quantity detecting device and results of detection by the second light quantity detecting device.
Thus, even when variations occur in the optical characteristic of the extracting device which extracts at least part of the light beam in the adjusted light quantity, or when foreign matter adheres to the extracting device and causes variations in the light guiding efficiency in guiding the extracted laser beam from the extracting device to the first light quantity detecting device, resulting in variations in the light quantity detecting accuracy of the first light quantity detecting device, the accuracy of the first light quantity detecting device in detecting the quantity of the laser beam based on results of detection by the second light quantity detecting device of the quantity of the laser beam emitted to the image-receiving object. Thus, the quantity of the laser beams emitted to the image-receiving object may be adjusted to a proper value, to maintain high-quality, high-definition drawings requiring a high degree of light quantity precision.
Preferably, the second light quantity detecting device is disposed in an imaging area on the holding device. The laser beam emitted from the exposing device impinges upon the second light quantity detecting device inside the imaging area on the holding device. The second light quantity detecting device detects the quantity of the laser beam. Thus, where the second light quantity detecting device is disposed outside the imaging area on the holding device, the primary scanning range of the laser beam must be enlarged so that the laser beam directly enters the second light quantity detecting device. This requires a large lens and large deflector to be used in the exposing device for scanning the laser beam in the primary scanning direction relative to the image-receiving object supported by the holding device. According to the invention, the second light quantity detecting device is disposed inside the imaging area on the holding device, and thus no need to enlarge the primary scanning range of the laser beam to direct the beam to the second light quantity detecting device. This avoids the enlargement of the lens and deflector used in the exposing device.
Preferably, the exposing device is a device for emitting a plurality of laser beams toward the image-receiving object held by the holding device. The drawing control device is arranged to control the exposing device and the moving device for directing the laser beams, one by one, to the first light quantity detecting device and the second light quantity detecting device. The first light quantity detecting device and the second light quantity detecting device are arranged to detect quantities of the laser beams, one by one, respectively. The calibrating device is arranged to calibrate the first light quantity detecting device for each of the laser beams based on results of detection of each of the laser beams detected by the first light quantity detecting device and the second light quantity detecting device. Thus, the accuracy of the first light quantity detecting device in detecting the quantities of the laser beams may be calibrated also in the laser drawing apparatus for performing multi-beam drawing by scanning a plurality of laser beams over the image-receiving object supported by the holding device. The quantities of the laser beams emitted to the image-receiving object may be adjusted to a proper value, to maintain high-quality, high-definition drawings requiring a high degree of light quantity precision.
Preferably, the calibrating device is arranged to determine whether a difference between a first light quantity value detected by the first light quantity detecting device and a second light quantity value detected by the second light quantity detecting device exceeds a permissible value, and to calibrate the first light quantity detecting device when the permissible value is exceeded. Thus, whether a calibration is necessary or not may be determined simply.
In another aspect of the invention, a laser drawing method is provided for drawing a desired pattern on an image-receiving object by scanning the object with a laser beam, the method comprising:
a first detecting step for operating an extracting device to extract at least part of the laser beam in a quantity adjusted by a light quantity adjusting device, and operating the first light quantity detecting device to detect quantity of the extracted part of the laser beam;
a light quantity adjusting step for adjusting the light quantity adjusting device based on results of detection obtained in the first detecting step;
a second detecting step for operating a second light quantity detecting device to detect directly a substantive quantity of the laser beam reaching the image-receiving object after the quantity is adjusted by the light quantity adjusting device; and
a calibrating step for calibrating the light quantity adjusting step based on results of detection obtained in the first detecting step and the second detecting step.
According to the method of the invention, the first detecting step is executed for operating the extracting device to extract at least part of the laser beam in a quantity adjusted by a light quantity adjusting device, and operating the first light quantity detecting device to detect quantity of the extracted part of the laser beam. In the light quantity adjusting step, the light quantity adjusting device is adjusted based on results of detection obtained in the first detecting step. In the second detecting step, the second light quantity detecting device is operated to detect directly a substantive quantity of the laser beam reaching the image-receiving object after the quantity is adjusted by the light quantity adjusting device. In the calibrating step, the light quantity adjusting step is calibrated based on results of detection obtained in the first detecting step and the second detecting step.
Thus, even when variations occur in the optical characteristic of the extracting device which extracts at least part of the light beam in the adjusted light quantity, or when foreign matter adheres to the extracting device and causes variations in the light guiding efficiency in guiding the extracted laser beam from the extracting device to the first light quantity detecting device, resulting in variations in the light quantity detecting accuracy of the first light quantity detecting device, the accuracy of the first light quantity detecting device in detecting the quantity of the laser beam based on results of detection by the second light quantity detecting device of the quantity of the laser beam emitted to the image-receiving object. Thus, the quantity of the laser beams emitted to the image-receiving object may be adjusted to a proper value, to maintain high-quality, high-definition drawings requiring a high degree of light quantity precision.